The contamination of surface and near-surface soils has become a matter of great concern in many locations in the United States and other countries. Increasingly the public has become aware of spills and landfills of toxic and hazardous materials in the soil. If left in place, many of these contaminants will find their way into aquifers or into the food supply, and could become public health hazards.
There are many proposed methods for removal of surface contaminants, such as excavation followed by incineration, in-situ vitrification, bacterial attack, chemical additives for deactivation, radio frequency heating, etc. Although successful in some applications, these methods can be very expensive (hundreds of dollars per ton) and are not practical if many tons of soil must be treated. Moreover, these methods become very expensive and impractical if the depth of the contamination is more than a few feet. The Environmental Protection Agency (EPA) has targeted over a thousand sites for Superfund cleanup. Yet, to date, the EPA has partially cleaned up only a fraction of these sites, spending a staggering $4 billion in the process (U.S. News & World Report, Feb. 6, 1989, p. 47-49).
In U.S. Pat. No. 4,670,634, Bridges et al propose a method for in-situ decontamination of spills and landfills by radio frequency heating. The Bridges et al patent proposes heating the region with bound radio frequency energy (preferably 0.5-45 MHz) from a boundwave transmission line excitor disposed above the soil. The soil is heated to a temperature higher than that needed to boil water, which is said to increase the permeability of the region. A vapor and gas collection and containment barrier is installed above the region to be decontaminated. The heating is continued by dielectric heating after water has boiled from at least a portion of the region so as to heat the portion to elevated temperatures substantially above the boiling point of water. The material is rendered innocuous in-situ in a number of ways, as by pyrolysis, thermally assisted decomposition, distillation, or reaction with an introduced reagent, such as oxygen. The materials may also be driven from the region, as by distillation or by evaporation and steam drive and then collected and disposed of by incineration.
While U.S. Pat. No. 4,670,634 is a significant advance relative to previous remediation methods, it has several disadvantages. The use of radio frequency power in the MHz range makes the process hard to control as the soil dries out; results in less uniform heating because of "hot spots" which are overheated every half wavelength and "cold spots" which are underheated; results in loss of efficiency in the generation of radio frequency power; and emits electromagnetic noise which can interfere with radio communications. Also, because the process in U.S. Pat. No. 4,670,634 operates at atmospheric pressure it requires cumbersome vapor collection barriers at the surface.
What is desired, therefore, is a method for effectively removing contamination which avoids the excavation of large quantities of soil; which can be utilized at depths greater than a few feet; which will efficiently and economically operate at temperatures of no more than about 100.degree. C. while removing contaminants with boiling points substantially greater than that of water; which is generally applicable to a wide range of contaminants (even those with low vapor pressure at room temperature); and which can reduce the level of contamination down to low levels throughout the treated interval.